1. Technical Field
This invention relates to semiconductor devices, and more particularly to test structures formed in a surface of a semiconductor device for detecting mobile ionic contamination at a silicon-to-oxide interface, during manufacture.
2. Description of the Related Art
In the manufacture of semiconductor devices, test structures are added to the circuitry on a chip or wafer to facilitate making various tests. One of the qualities for which test data is needed is that of the degree of contamination by mobile ions at a silicon-to-oxide interface. Particularly, ionic contamination is an issue in MOS field-effect transistors where the junction between the gate oxide and the underlying silicon channel region is susceptible to this problem. Whenever an electric field is maintained across a silicon oxide layer for long periods of time, ions of contaminant materials will migrate toward an interface. The time for this contamination to build up to a point where the threshold voltage is noticeably affected is lengthy, however, so a life test cannot be performed in any efficient manner at the time of manufacture. Accordingly, various test structures and methods are used to speed up the ionic migration so that a valid test can be made in a reasonable time. The ionic migration is dependent upon voltage level, i.e., the electric field applied across the gate oxide, and is also dependent upon temperature. So, test structures and methods have relied upon applying higher-than-normal voltages to the devices, and on heating them to above normal operating temperatures. In this manner, tests may be made in much shorter times.
Previous methods and test structures have relied upon a slotted polysilicon heater to allow the mobile ionic contaminants to move more rapidly in the presence of an applied bias. The test structure is essentially a transistor, the source and drain regions being N-well diffusions and the gate electrode being a metal plate. The gate dielectric in that case was the full thickness of the dielectric below the metal plate down to the silicon substrate. Because of the polysilicon heater, the effective width of the transistor is the width of the slot in the poly heater, and the transistor width is the distance between the N-well regions. In order to maintain the temperature in the region through which the mobile ions must pass, the slot in the polysilicon heater must be restricted to a fairly narrow width. This narrow width restricts the sensitivity of the test structure.
It is therefore preferable to eliminate the polysilicon heater in a test structure for mobile ion contamination, so that greater sensitivity can be obtained.
It is therefore one object of the present invention to provide an improved method of testing semiconductor devices.
It is another object of the present invention to provide an improved method of manufacturing MOS devices in which detection of mobile ionic contamination is more efficient.
It is yet another object of the present invention to provide a method of detecting mobile ionic contamination in MOS devices which is more sensitive than previous methods.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.
According to one embodiment of the invention, a test transistor structure formed in a semiconductor device has a thick-oxide transistor with an elongated serpentine-shaped metal gate. The gate is used to first measure the threshold voltage of the thick-oxide test structure. Then, a current is passed through the elongated metal line which forms the serpentine gate to heat the area of the test structure. While being heated, a stress voltage is applied between the substrate and one end of the gate electrode, this stress voltage being much larger than the logic voltage used in operating thin-oxide transistors on the chip. After a selected time, the current is removed, the stress voltage is removed, and the threshold voltage of the thick-oxide transistor is again measured and compared to the original value. Any reduction in threshold voltage can be attributed to the migration of positive charge to the silicon-to-oxide interface beneath the gate, and is proportional to the area between the source and drain regions of the test transistor. The improved test structure is thus a thick-oxide transistor with N+ source and drain regions and a serpentine-shaped metal gate. The high temperature for the stress is supplied by Joule heating in the serpentine metal line which is between the source and drain regions. The line is long enough to permit accurate resistance measurements that are used for temperature determination. Because of the thermal gradient between the Joule-heated metal line and the substrate, there is an additional directional drive on the mobile ions. In this structure, nearly the entire length of the metal line can be used to detect mobile ionic contaminants.